In a shaft-to-shaft driving gear mechanism, a chain or a toothed belt mesh with driving gears is used.
However in these mechanisms, the chain or the belt flutters or vibrates in addition to rotation at a constant speed. When the speed increases or varies, noises are generated, or the chain is often broken. Especially the chain much makes such troubles caused by abrasion expansion.
To prevent noises and/or chain destruction, in general, tensioners are installed so as to apply suitable pressure to the chain or the belt. The tensioners are divided into two main types. One of them is a no-back system which avoids only loosening of the chain and comprises a pressing element arranged not to return to an original place, and the other is a damper type which mainly prevents fluttering and presses the chain or the belt only if required.
FIG. 3 shows a damper typed tensioner 100 positioned against a timing chain 30 of an internal combustion engine, and FIG. 4 shows a structure of a hydraulically operated damper type tensioner. This tensioner 100 is provided integrally with a hydraulically operated mechanism built in a main body thereof. A detailed explanation is made hereinafter.
The tensioner main body is composed of a tubular frame body 110 which is fixed at a required position for supplying a fluid such as a lubricant via a liquid path 110a, and a tubular projection member 111 which is slidably fitted into the body 110 and whose closed end forms a slip face 112 to press against a chain 30 and whose other end is open.
A hollow space defined between the projection member 111 and the frame body 110 is served as a high pressure chamber 113 where the operating liquid is filled from the path 110a, and an elastic member 114 is expanded, and further a check ball valve 115 and a cage 116 are furnished, the former opening and closing the liquid path 110a and the latter supporting the former, whereby a hydraulically operated mechanism of the tensioner main body is built.
The timing chain 30 slides directly on the slip face 112, and if the chain 30 flutters and hits the slip face 112, the tensioner main body is displaced. At the same time, when the operating liquid filled in the high pressure chamber is subjected to pressure, the operating liquid is compressed and then the tensioner main body is not further displaceable.
On the other hand, when the timing chain 30 is separated from the slip surface 112, and the pressure is released, the elastic member 114 within the chamber 113 is expanded, and the tensioner main body is extended in an opposite direction. Thus the damper typed tensioner 100 is compressed and its length is reduced by the operating liquid when the pressure is applied to the operating liquid.
The elastic member 114 extends the tensioner when the pressure is released, so that the slip surface 112 follows the fluttering or vibration of the timing chain 30 and damps it.
However, in the tensioner having the above mentioned structure, while the chain or belt slides on the slip face 112, a large side load is applied to the sliding side surface between the tubular frame body 110 and the projecting member 111. Under such a condition, if fluttering of the chain or the belt, the abrasion of the sliding side surface is accelerated and causes problems such as bitings between the body 110 and the member 111 when the projection member 111 moves upward. If the abrasion of the sliding surface grows, a clearance between the body 110 and the projecting member becomes uneven, and the operating liquid leaks substantially from the high pressure chamber 113 and adversely affects the operation by the liquid pressure of the tensioner 100.
If the tubular frame body 110 or the projection member 111 is formed with a hole for purging outside the air from an aerated liquid, the liquid escapes through this hole, and therefore the air hole cannot be formed in the tensioner main body. Thus, the air mixes easily with the operating fluid, and the damping effect is lowered in the above conventional structure.
For these reasons, the flutter preventing function is lowered in the conventional damper typed tensioner, and the chain or the belt hits the slip surface 112 and makes big noises. The worst matter is break of the chain or the belt.